This image posted Wednesday by the Raqqa Media Center of the Islamic State group, a Syrian opposition group, shows a munitions warehouse that was captured by the Islamic State group in the battle for the Tabqa air base in Raqqa, Syria on Sunday.

BEIRUT — The mother of a hostage American journalist pleaded for his release Wednesday in a video directed at the Islamic State group, while new images emerged of mass killings, including masked militants shooting kneeling men after the capture of a strategic air base in Syria.

Shirley Sotloff’s plea came as a U.N. commission accused the group, which dominates a broad swath of territory spanning the Syria-Iraq border, of committing crimes against humanity and President Barack Obama weighs options for targeting the extremists’ stronghold in Syria.

The Islamic State militants have threatened to kill 31-year-old Steven Sotloff unless the U.S. halts its airstrikes against it.

Sotloff, who free-lanced for Time and Foreign Policy magazines, had last been seen in Syria in August 2013 until he appeared in a video released online last week by the Islamic State group showing the beheading of fellow American journalist, James Foley. Dressed in an orange jumpsuit against the backdrop of an arid Syrian landscape, Sotloff was threatened with death unless the U.S. stopped airstrikes on the group in Iraq.

Addressing the leader of the Islamic State by name, Shirley Sotloff said her son was “an innocent journalist” who shouldn’t pay for U.S. government actions in the Middle East over which he has no control.

Speaking directly to Islamic State leader Abu Bakr al-Baghdadi, who describes himself as a caliph, or Islamic leader intending to lead the Muslim world, she implored him to show mercy and follow the example of the prophet Muhammad.

“You, the caliph, can grant amnesty. I ask you, please, to release my child. I ask you to use your authority to spare his life,” Shirley Sotloff said on the video, which was first aired on the Al-Arabiya television network. It was widely retweeted by Islamic State supporters later Wednesday with her face blurred because their ultra-conservative interpretation of Islam doesn’t allow a woman’s face to be shown.

At the White House, spokesman Josh Earnest told reporters he did not know whether Obama had seen Shirley Sotloff’s video appeal, but he said the administration was “deeply engaged” in trying to gain release of all Americans held hostage in the Middle East.

“She obviously, as is evident from the video, feels desperate about the safety and well-being of her son, and understandably so, and that is why our thoughts and prayers are with Mr. Sotloff’s family at this very difficult and trying time,” Earnest said.

In Geneva, meanwhile, a U.N. commission accused the Islamic State group of committing crimes against humanity with attacks on civilians, and photos emerged of the extremists’ bloody takeover of a Syrian military air base.

In one photo posted online, masked gunmen were seen shooting seven men kneeling on the ground, some dressed in what appeared to be Syrian military uniforms, after the seizure of the Tabqa air base in the northeastern Syrian province of Raqqa earlier this week.

The images underscored how the group uses violence, and images of violence to terrorize its opponents, as it sweeps further into Syria and Iraq, where it has imposed an Islamic state, or caliphate, governed by its harsh interpretation of Islamic law.